“I think it’s recognition of the fact that a) circumstances have changed within the eurozone with the migrant crisis washing up on EU shores and b) the fact that Brexit is a much more real possibility than many EU leaders had anticipated.”

Brussels officials describe it as a ‘crucial weekend.’

A draft package could be circulated to EU governments as soon as Monday, if agreement can be reached over a new ’emergency brake’ – which would let any state cut benefits to workers arriving from the EU, if other leaders agree its welfare system is at risk.

“I think there’s a very real risk that it certainly could go wrong, yes. And if we end up with a referendum this year without having had a proper debate about what are the costs and benefits of the EU then I think yes we could well sleepwalk into Brexit.”

A referendum must be held by the end of next year.

David Cameron will also meet with European Council President Donald Tusk on Sunday evening.